This invention relates generally to a method of detecting a mark by an electron beam. More particularly, the present invention relates to a method to be used for detecting with a high level of accuracy the position of a mark for positioning provided on a sample face in an apparatus for electron-beam lithography and the like.
Mark detection systems in an apparatus for electron-beam lithography are generally directed to carry out positioning of a sample (mask or wafer) and measurement of the deflecting distortion of a beam system.
When a region formed by depositing a gold (Au) evaporation mark 3 (or a silicon, recessed step mark) onto the surface of a silicon (Si) substrate 2 is scanned by an electron beam as pictured in FIG. 1A, there is obtained on a solid state detector 4 a detected mark signal 5 due to the reflected electrons as shown in FIG. 1B.
A well-known fundamental method of detecting the position of a mark comprises amplifying a mark signal produced as the output from the solid state detector 4 to a suitable level, binary-coding the amplified signal at a certain threshold voltage, measuring the time from the initiation of scanning until the generation of the binary-coded pulse, and determining the position of the mark relative to the scanning starting point from the scanning speed of the beam and the measured time. In other words, assuming that the scanning speed of the beam is v (m/sec) and the time required for the binary-coded pulse signal to be obtained is t (sec), it is possible to find out that the position of the mark is separated by a distance of v.t (m) from the starting point of the scanning.
It is possible, in principle, to detect the position of the mark in accordance with the above-mentioned method. However, a number of problems occur in actually practicing this prior art method. For example, in using this method there is a lowering of the signal-to-noise ratio (S/N) of the detected mark signal. The degree of this lowering of S/N naturally varies depending on the fluctuation of the beam current emitted from an electron gun, the fluctuation of sensitivity characteristics of the solid state detector, the shape and size of the mark, the material of the mark and so forth. In any case, however, S/N is definitely lowered using this method.
Another problem in using this method is that it frequently happens that the accurate mark position cannot be detected when the binary-coded pulse signal is generated at a position different from a predetermined position because impulsive external noise generated by other instruments mixes with the detected mark signal. The impulsive noise can be removed to some extent by using a reinforcing electromagnetic and electrostatic shield, but such removal is not complete. At the same time, since S/N of the detected mark signal is inferior, it is difficult to determine a threshold level of a binary-coding circuit where a number of pulses occur at positions other than the predetermined position and accurate detection of the position becomes unfeasible.
To solve these problems, there has conventionally been practiced a method which scans the position of the same mark a number of times, say, 100 times, and determines the mean value as the position of the mark. According to this method, however, the positioning-detection is not only time-consuming but also the accuracy of detection is not entirely satisfactory.